Which of these numbers is not divisible by 9?

1. 729

2. 486

3. 634

4. 891

A number is divisible by 9 if the sum of its digits is divisible by 9.

1. 729 → 7 + 2 + 9 = 18 (divisible by 9)

2. 486 → 4 + 8 + 6 = 18 (divisible by 9)

3. 634 → 6 + 3 + 4 = 13 (not divisible by 9)

4. 891 → 8 + 9 + 1 = 18 (divisible by 9)

Hence, option 3 is the correct option.